Add container registry online GC SLI
Context
The new container registry backed by a metadata database has been deployed to pre-prod (gitlab-org&5392 (closed)) and we're now working on adding as many metrics as possible to the registry Grafana dashboards as part of https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/12795.
Changes
This MR adds an online garbage collection SLI detail section to the registry overview dashboard:
Edited by João Pereira